You might try some black ABS cement and sheet of fiberglass.
Had some cracks from the package rack on the GW1500. Picked up a 3'x3' sheet of fiberglass and a can of black ABS cement at Lowe's. (The tupperware on the 1500 is ABS - same on the 1800? Beats me.)Drilled a very small hole at the end of the crack for stress relief and glued the cacks from the inside. After gluing the cracks, filling in the holes with the ABS cement (the black stuff has a solvent with ABS plastic dissolved in it) and touching up the paint I painted the inside of the trunk with a good layer of the cement. I then pressed a layer of fiberglass into the cement. If all the fiberglass didn't turn black I brushed a little more cement on It. This is just like laying on fiberglass with ABS cement instead of epoxy. I built up the inside of the lid about 1/8", took a few layers of the glass mat. That thing is a lot stronger now, and I have no worries more cracking or using the trunk rack. Fixed a buddy's black 1500 trunk that had a crack along the ridge just above the side light. Can't even tell it's been cracekd. We did the other side and beefed up the trunk just for insurance, haven't had any more troubles.
A few precautions - you are dealing with solvent- wear gloves, use a brush you plan to throw away for spreading the cementworing the fiberglass. Make sure there is plenty of fresh air. Don't rush, but the stuff sets up pretty quick, so plan out the job and cut the matting in advance.