Good idea... father had a bed like yours without the opening, after 1st summer he also added a support, they felt like the bed was sinking in the middle.
My JayFlight 28BHBE came with the support closed off
2014 JayFlight 28BHBE (replacing a 2009 Kodiak 25QS)
2011 Sierra SLT CrewCab 4x4
1200/12,000 Equal-I-Zer Hitch