I've built the bottom half of a fiberglass enclosure and now need to build the top piece. My plan is to take one .5" mdf piece and join it with a .75" piece roughly the dimensions of the enclosure so I can flush mount the sub. This piece will measure roughly 16 x 19 and the holes will be cut for the flush mount. Then I will glue it in place using mdf pieces cut accordingly so I can glass the top piece to the bottom. My problem is the top piece needs to slope down at an angle and I don't know what angle to cut the mdf pieces that will hold up the top piece. The highest point is 10"/8.25" externally and internally. The lowest point is 8.5"/ 6.75 respectively. Additional info, the enclosure is 19"/18.5" long respectively. If anyone can help me with this I would really appreciate it.