I have used an automotive type flat black rust paint which is available locally and comes as a spray can application or in a 237ml can for brush. Also have completed upgrades as suggested originally by @stoft with some modifications to suit my purpose one of which I tapped a 13/64" - 5,16mm hole with a 1/4-20 tpi thread to mount the spectrometer onto a tripod. Also I used some flat black cardstock to line the interior and just painted the areas that were not convenient to line with paper.
This shows a 1/4-20 metal adapter for a tripod mount also addition of the rear 1.5X2 bulkhead with an indicator power on lead for the scavenged web cam and @stoft drawing.
This image is the next project showing the front porch for the cuvette holder , the tappped into maple tripod thread, various camera support blocks and the paper insert for reflection suppression. Not shown is the paper cover.