Mounting small class D amp to pedalboard?

Check-out the main Quilter thread. A few users are doing that with the Bass Block.

Quilter Bass Block

 

As long as it gets good air flow and it's protected when when you travel, why not? The manual might state minimum air space requirements.

 

I thought about the same thing a lot.
My two main problems were that
1) I like my speaker cables as short as possible. Usually I make new ones when I get a new cab or head. With the poweramp on your pedalboard you will need to use a fat, unwieldy and unshielded cable in different settings - so you will have to have a rather long one to accommodate for all eventualities (Cab right next to the board vs. board in front, cab behind you et cetera).
2) I don't like to have a poweramp on the floor where it gathers dust and dirt in terrifying speed and is in constant danger of getting showered in liquids. Get a glass of beer on your pedalboard and you'll most likely hear a loud pop.
Get a glass of beer on your pedalboard with poweramp and the pop will sound different. I'm no engineer, but I guess all those watts can get pretty angry.

I use a small class D Poweramp right next to whatever speaker I'm driving, that solution feels better for myself.

 

Pouring your beer on you pedal board is alcohol abuse.