Timber thickness for seating platform

[pike]

I have decided to start (and this time finish) another project around the home. I want to build a removable/temporary seating platform that will rest on my spa bath in our upstairs bathroom.
We have a new baby on the way and have realised that the spa bath is too deep to keep bending down to for babies bath time so I thought I could cap off the tub with a makeshift deck so that my wife or I could sit on and place the baby bath on top of to make thing easier. We dont use the spa we just use the shower so there is no worries of continually removing the cover, it will be a once on, once off job (hopefully off will be in a few years)

Question:

Since the deck will be supported on all sides by the bath rim, how thick should i make the frame and cross pieces to support around 100kg (worst case with me, a baby, a small tub of water and ten thousand baby washing products)


I have linked to a pic of a spa bath similar to ours, the setting is exactly the same but the controls are on a corner instead of the middle of one side (dont think thats relevant anyway since we wont be using it).
http://www.johnsonroadmotel.com.au/i...r_Spa_Bath.jpg

[Brobdingnagian]

what is the maximum span across the tub?