915 is mounting

rotax-owner.com has an excellent series of videos on mounting a 912iS to a Zenith CH750. Watching all 10 installments will give you a pretty good idea of what needs to be done and at least get you pointed in the right direction.

Does Just Aircraft offer a cowling option specifically for the 915iS engine? How about an engine mount?

Are you building an XL or a shorty?

https://www.rotax-owner.com/en/videos-topmenu/912-is-videos

Refer to the 915is Illustrated Parts Catalog,  Section 71-20-00, Engine Suspension Frame, as a starting point.
One Rubber Shock Mount (Item #10) goes on each side of the aircraft engine mounting framework. (Not shown)
The Struts (Item #11) go through the middle of everything, and the mounting Bolts go through the Strut.
And the Cup Washers (Item # 9) sandwich everything together.

Google "Aircraft Lord Mounts" for example images and videos

Make sure you contact Just Aircraft to get the correct mounting hardware.  They do not use the Rotax bolts and rubber mounts.  We mounted a 912iS with Ring Mount onto a Just Highlander and had similar concerns.  There are several 915iS guys on the Just Aircraft Facebook group.