Hoosac has now been here long enough for a pattern to emerge: He always poops and pees in his warm hide or right outside it, and then sits in said warm hide when he has done so. He uses his other hides plenty, so it's not like he only wants to go into that one. But apparently after he has made a mess in it, that's where he wants to be.

I have aspen as substrate in most of the tank, but just bare paper towel where the warm hide is to keep the heat from the heat mat more even/reliable. But the paper towel means that the urine spreads out over a wider area than if he would just pee on the aspen; so if he pees at the entrance to his hide and then sits in his hide, he's still sitting in his mess.

Obviously I clean things up the first chance I get, but he's still sitting around in it for awhile before that happens. I wouldn't worry too much if it just happened occasionally, except that so far he has managed to do that every single time without fail.
Any suggestions for how to keep him from sitting around in pee every time he goes? Another identical warm hide? Thicker substrate (with the concomitant necessity of the heat pad being warmer)? Warmer ambient temp so he won't want to go into the warm hide as much? Warmer hot spot so he'll go there for a shorter stay? A tiny litterbox with a sign that says "please flush"?

The tank is a 20L wrapped in foam with an 80-watt RHP on the lid (yes, it takes up most of the lid except for about 3" on each end, but in the winter our living room is usually between 55 and 65 degrees) controlled by a herpstat and set at 82° for the ambient temp. I set it at 82 instead of 80 because the front corners, which aren't under the RHP, are a little cooler and he has a hide in one of those spots, so he has a cooler hide if he wants it. The warm hide is about 88 for the air temperature with the surface temp of the floor a couple of degrees higher.