I finally found a pair of upper struts for my D90 fiberglass hardtop at Lift Supports Depot on eBay:
(2) 10mm nylon end lift support 23" extended by 30 Lbs struts gas springs
Manufacturers number: SE 230P30
Item number: 231919734766
$29.80 for 2
They're installed and work perfectly.
Thanks to JPerakis for the lead!
84 110 Dormobile
94 D90 ST