Why not just get a bigger UPS and avoid the liability of making your own? The risk is real.

I use a high-end Dell laptop which has as internal battery so all I needed was something to continue powering the battery to extend on-time. For that purpose I have a Xantrex 400 Plus. It's available in bigger sizes such as the Xantrex XPower Powerpack 600HD. The Xantrex 802-1500 XPower 1,500 Watt Portable Powerpack is probably a little bigger than necessary. All of these units can output 120VAC or 12VDC.

Duracell markets similar units such as the Duracell DPP-300EP Powerpack 300.