Re: 1800 Adventure Bimini
I think the correct name Lund has for yours is a top set configured as a Sun Top, and yes there are side and slant curtains you can get for it. The slant curtain covers the rear of the cockpit area.
Lund also offers a Bimini Top, one where the front does not attach to the top of the windshield, but is open instead to let in more air. It covers more square footage of the cockpit than the Sun Top.
We have had our boat on plane at around 19 mph with the Sun Top up with no problems. Could probably go faster, I suppose. Just make sure all the the snaps are secured on the windshield. If you get the optional Bimini Top I would be hesitate to get her up to speed as the force of the air underneath might tend to lift it off its supports.