Buy 2 sheets, and use one cut up into 8" wide strips that are underlay-ed(glued) around the edges to reinforce them.
As for the White Gloss, it can be your best friend. be it BW or SF, or even a stock neutral Gray, when the right amount is covering that Gloss, (enough to impart a even Gray hue....but "JUST" enough!) absorbed light that has been attenuated through absorption will reflect back to infuse the initial surface reflection with a significant boost across ALL the spectrum, from whites / off whites to Grays, to light hued to deep dark colors. It's a balance you seek, and if balanced, it's all good.
This sort of "White Fusion" is essentially "First Surface" because the Gloss White layer is the surface the Top Coat over lays. The reflection interaction between a painted Top Coat and the Gloss Underlayment takes place at the Micron-level as a transfer of energy from one surface to another (Top Coat to Gloss Coat) and that results in a Radiant Reflective response (Gloss Coat to Top Coat) that reintroduces the same light, only attenuated to whatever degree the combination of absorption/reflection provides.
As long as the reaction is not excessive due to too much reflection (...as in too thin a Top coat) or completely muted altogether (...as in too thick a Top Coat) the end result will alway mean a marked improvement.
Whats to be considered "Glossy"?
But when you paint it correctly, it becomes this......
And then you get to see this sort of thing..........
So wish upon a Star..........or several hundred.