I'm looking for a good place to mount a ham and, possibly at some point in the future, CB radio in my truck (F-250).

The radio is a Yaesu FT-8900R, which does have a remote-mountable faceplate.

I used to mount it near the transfer case shifter with it's "quick-release" brackets. That made it easy to remove/replace, gave it good airflow, and kept it off the floor.

Now, though, I have a mobile computer station there, and there's simply no room for it.

I thought about mounting the radio under the driver/passenger seat and using the remote faceplate, however, we do a lot of trips to the beach, and that means moisture/sand is inevitably going to make a presence there.

I do have one of the Du-Ha organizer bins under the rear seat, that would at least offer some protection from moisture/sand, but it will inevitable get stuff piled on top off it in those bins.

I also mulled mounting it under the driver/passenger seat in a waterproof "project box," but the trade off there is that it would really be starved for airflow and might overheat...

Water and sand fall down, so even a thin sheet of plastic over the radio will protect it from 99% of the stuff,
especially if it's off the floor a couple inches.

My 7900 lives under the passenger seat of my jeep and has for 10 years. No issues and it doesn't even have plastic over it,
just the seat.

It is pretty dusty now, though.

What about mounting it to the back of a seat?  When I worked in car audio, we used to mount amps to the back of the seats in single cab trucks all the time.

I've got an XTL5000 that's been hiding under the passenger (rear) seat of my Sierra for 7 years now. Never had an issue with it (it's a high power unit so it's in a tray and not mobile trunnion bracket). I've got another XTL5000 (mid power) that sits under the back seat of my XJ and has been there long enough to survive Texas summers and Colorado winters and has also never had an issue (there is zero air flow under that seat but the pan is sloped righter there to transition from the cargo area to the footwell so it does drain. Had that Jeep for nearly 14 years now, have never had water come into the vehicle that wasn't snow melting off of something I put into the back (granted, I don't have carpets and sound insulation to retain moisture...just bedliner). I will say I checked on that XTL a few months ago and it had a decent layer of dust on it (a few years later). Hit it was shop vac with brush attachment and it's good to go.

I'm running an FT-8900 in my Transit Connect now, but did have it in my last F-150 with pretty much the same setup. I was pretty hard on my F-150; 242,000 miles and the cab body was rusting out from running in tidewater dirt roads full of brackish puddles. I hunted, fished and trained out of it and never really did much of anything except dump the floor mats occasionally. If I opened the windows at highway speeds, I'd get dirt in my eyes. Same radio has been in it the entire time.

I have the radio body HUNG under the driver's seat. I used heavy-duty tie straps to pull the bracket up tight against a crossbar under the seat. The radio hangs under the mounting bracket and it's up out of the way with airflow all around.

The control head is on a gooseneck seat bracket mount by Lido, similar to this one (https://www.lidoradio.com/collections/motorola-radio-mounts/products/car-floor-seat-bolt-mount-for-uniden-sds100-and-bcd436hp-scanner), except the top connection is different. The gooseneck is a solid soft aluminum rod (3/8"+/-), covered in black corrugated wire loom, that bends to the desired spot and stays put.

There is a pair of lugs on the top of mine that engage the slots on a 'universal plate' that bolts to the back of the control head, like a quick-disconnect. I have the bottom attachment point of the mount bolted to the front-right corner of the seat rail. The seat won't pull completely forward anymore (lacks about 2"), but I need it all the way back anyhow.

Be careful if you have electric seat adjustments - might need to reset the limits. Mine was/is manual adjust, so I never broke anything.

The top end of Lido mount has a tension-adjustable ball joint and so the radio control head has a wide latitude for adjustment. I got a similar Lido mount that uses my dysfunctional CD player slot as an attachment point and I have the same lug/slot arrangement with that and the back of my Garmin. I can put the radio or GPS in either place. If I anticipate using the radio a lot, I'll move it up to the higher CD slot and it's more eye level. If I need GPS to navigate, I leave the radio control head between the seats/in front of the console on the seat bracket mount and place the GPS closer to eye level. I drop the mic into a cup holder, but really should have a clip someplace.

When I was driving a Tahoe with a metal center console, I mounted my Yaesu 7250 vertically on the console between it and the driver seat.  I had to be careful not to drop crumbs or liquids on the front of the radio (especially with a front-firing speaker) but the heat sink and cooling fan on the back of the radio had plenty of air flow.  It worked very well but I never took a picture of it.