Hi,

Your best bet is to go look around in a junkyard for the screws and there are two plastic cups that the screws fit into.
Black ones are the most common but they are in other colors but the sizes stays the same.

The whole assembly is difficult to see when the seats are in place so you don’t need to be picky about color and black is very common. If you have a brown cover the black cups can be considered highlighted accents. (:-)

In 1991, the screws could be either a Phillips head or a Torx Star head and in a black oxide color.
The nineties Volvo’s started getting the Torx Star Specials, everywhere! Well, almost everywhere?
It’s been awhile since I have pulled mine out, but I think they are a common sheet metal screw size?

You might find them still attached in a “broken up cover” that nobody wants?
They will probably still be attached at the rear portion along with the ashtray assembly, if you need one.
The screws will look recessed down into the cups when installed as the cups are only about 10mm long. They have an upper lip that catches on the covers side and it provides the boss to fit under the screw head. Otherwise, they would to have had to mold it in place and that would have complicated the ejection operation of the part from the mold.

Look around in the floorboard areas and under any carpets or mats. I have seen both cups and screws that do get left behind all the time.
Most of the time some other item is snatched, like the lights bulbs and complete switches for seat heaters. If the cover is already damaged they just go to ripping and busting!

The cover will just sit in place, most of the time, without those fasteners until you find some!

I know it it is worrisome, not having it right, but you know how that goes on a project cars!

Phil