A question about alternative power supply for hp41 series

[wojtek]

        I enclose a picture of mine HP41c powered by a converter 12-5.5V and single nimh AA accu. The setup is very simple and provisional for a while, the capacitor on the right side has 100uF. I do not know yet what is the power consumption and how long the accu will last. The converter used is a backup power up of a telephone and can provide some 600ma max. The nimh accu used had about 1.1V.
On the second picture the electronic circuit is shown. It is very small, about 16mm in diameter and will fit in the battery holder of 41C without problem (of course without the aluminium body shown in the picture) along with a single nimh AA accu.
Electronic setup took some 10 minutes to assemble but the problem for me was in making the connector to 41C (after a few tries I decided to make it of the epoxy resin)
But in the near future I expect even greater problem in making the cover of the battery holder of the 41C. I would be grateful for any ideas how to make it.