i need remington 870 accessories for a 20g.

it's time to put together a truck gun for Girl. i was going to put together a 12g for her but i lucked into a buddy deal this morning and picked up an old magnum wingmaster 870lw in 20g for 75$.

this poor old gun looks like it's been through hell already so i'm gonna knock the surface rust off it and maybe duracoat it, not sure yet since it's going to be a truck gun anyway. i know for sure though that it's getting synthetic furniture, a side saddle, and a mag extension.

i'm pretty sure i can snag the stock and forearm from remington directly, i haven't made it that far yet. right now though the ass kicker is finding a mag extension for 2g, everything i'm finding is 12g only. where should i look for this thing? i know there used to be a tactical companie that catered to the 20g market but that was years ago and i can't remember the name now.