I have a few MOLLE pistol and rifle ammo pouches. For travel in public places I'd like to attach them to the inside of the bag so as to not draw attention to myself. Sure, I can buy a backpack with internal pockets but they aren't removable and can't be quickly adjusted to my needs.

Does anybody make a decent pack with MOLLE on the inside in the main compartment?

I've used an Eagle Industries side plate pouch for that. It's a flat MOLLE panel that can fit several magazine pouches. It isn't quite what you are asking for, but it could be placed in a more innocuous bag and moved as required.

You might get better responses in the Armory.  I know there have been some made with pals webbing on the insides, but a better solution is a backpack with loop material sewn inside, so you can use hook-backed pouches and just velcro them where you want.  Even if you want to use your current accessories, it is as easy as using the sticky-back industrial velcro and attaching it to the item.  Pals webbing is good, but the weight does add up, especially with cheaper packs made of cheaper materials.  Many of the better brands of packs will have the loop material in them.

The ATS Raid does. So did the original Cobra. I think the New Raid still uses MOLLE but the new Cobra uses Velcro.

My elberstock had the webbing on the inside.  I cut most of it out

You might also look into these. Would allow regular molle pouches you have to be added to any bag. Easily removed and inserted into another bag. They are also being sold by an ARFcommer.

ETA: I am not selling them nor do I have any affiliation with who is.

https://www.pelicanstrategic.com/

 
That's pretty badass. Would be cool if there were some larger sizes too.


 

ETA: RIF, looks like he does custom sizes Might have to send him an email.

I was going to suggest this.  I once had a setup made using a sheet of kydex type material.  The holster and knife sheath were fastened to it with Chicago screws.  Then tucked in to the water reservoir section of my back pack.  Sorry no pics though.

We do have the ability for custom sizes. The current RIPM panel was based on average backpack sizes. I use one in a Kelty Redtail 27 and LBT assault pack.

It looks like this can be cut to different shapes if desired?