Gun Cleaning Adapter.com
$6.50
Nylon Plug 3/4 NPT
Black Nylon 3/4 NPT plug
Black Nylon plug

Single-barb design provides a leak-proof seal.

Black nylon offers good tensile strength for this solvent trap application, with maximum pressure of 150 psi and temperature range of -50° to 275°F. Plenty.

Shipping is FREE and I highly recommend getting two plugs. You can then seal both the bottom and the top of your NAPA 4003 solvent trap when not in use. Or just to have an extra.