There's a guy from Germany who produces some nice c-core autoformers designed for use with Altecs, you can find him and the autoformers on eBay under the username mast_mutter. You can either buy a complete crossover network or order the autoformers and/or c-core inductors separately, his 604 network is very simple but works very well. PM if you would like his email address, otherwise you can contact him via eBay.