What about these @Rogerborg https://www.moaitoys.com/shop/index.php/cyma-handguard-pin-for-cm027-mp5-aeg-smg.html
Eagle 6 have Marui ones: https://eagle6.co.uk/shop/catalog/product/view/id/3191/s/tokyo-marui-mp5k-back-plate-take-down-pin-for-mp5k-standard-aeg-series/
https://www.moaitoys.com/shop/index.php/cyma-handguard-pin-for-cm027-mp5-aeg-smg.html