New England Custom Gun has several different models, you might want to look at.
They have traps with flat or curved lids, and spring-loaed lids, or lids that must be pried open.
Butt-end cartridge traps are generally easiest to install, but not as nice-looking as a well-done trap added to a buttstock's bottom line.
All I've done were just hinged covers of one kind or another - underneath which a series of proper-sized holes were drilled to accept the length and diameter of the user's cartridge of choice.
Either style requires that the buttstock be inletted for the hinged lid, and (if a bottom-line type) the stock edge flattened in the area under the lid.
Another butt-style trap can be handmade via reworking a buttplate's wood-side face for a thin sliding door, that can be withdrawn to open an oval cut into the BP's center section.
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