Something I've done in the past which works well for plecos but it's a little labor intensive; you'll need a pair(or two) of cheap surgical gloves(hardware store item), take a capers jar or any glass/ceramic jar/vase of the appropriate size, squeeze out a healthy glob of silcon on a piece cardboard then roll the jar in it covering the top surface (as it lays flat like it would in your tank . .you don't need to go all the way around the jar if you set it into your substrate a little bit) make it a thick layer. Then roll the jar in a pile of your dry substrate. Set up something so you can let this sit and cure. You can hand sprinkle (use a clean pair of gloves at this point) some gravel over any spots that need it and over the front edge of the jar so the glass doesn't reflect the light.
I've found some nice cheap ceramic and glass vases at GoodWill type places.
Justa thought