I'm not fully sure how past decrees work with current legislation and what may be "overridden" or not but if you want to go by previous decrees, this is what I thought off - someone else will probably fill in the gaps (and correct if wrong) Most of this information you'll find online in the CE - some of the post-1917 decrees and interpretations have slightly modified some parts and I've included those:
- The altar should be raised on at least one step (known as the predella). This is in accordance with the rubrics which speak of certain prayers being said before the steps of the altar, and also with the idea of ascending to the altar. The Caeremoniale mentions that a rug should cover the predella
Other requirements are dealt with in the General Rubrics 525-529
- According to the rubrics of the 1962 missal, the altar should be made of stone. If not made of stone it may have in it an altar stone which has been properly consecrated ** (When researching on this, you might find it helpful to look for the term Altari ad modum fixi
) By stone is meant natural not artificial like concrete.
- There must be a crucifix in the center with at least two candles on either side for a low Mass. The crucifix should be of a size that enables it to be seen clearly. A large cross (or painting of the crucifixion) on the wall behind the altar, in the middle would suffice. Likewise, a hanging cross in the middle was also admitted.
De Defectibus mentions that the candles should be of wax. One pre-conciliar requirement was that at least 2 candles should be of wax in "maxima parte" . This was defined separately by the bishops of each country- some said 65%, some said 70%, , 60% etc.
- There should be three blessed linen cloths. The highest must hang down to the ground on either side. Both the Pontificale
and Caeremoniale there was also mentioned another cloth called the chrismale - which is waxed - in the ceremonies for the consecration of an altar - not sure what happens to an already consecrated one without a cloth.
- Altar cards
- Technically, according to the Caeremoniale and liturgical decrees there should be a canopy over the altar extending at least over the middle and the predella. However, even before it was not common and I rarely see it in pictures of Traditional Masses celebrated by (formerly) Indult and non-Indult communities. Some liturgical manuals I have say it is only necessary for altars of the Blessed Sacrament. .
** Here would come the question of what is meant by "properly consecrated" (I don't know the answer). At the time of the 1962 missal, based on the inference of 1917 Code, the majority opinion held that the relics were an essential part for a validly consecrated altar or altar stone. There were those who did hold that it was not essential to have relics - and Indults were granted sometimes for countries to celebrate Mass on altars without relics. For a more detailed view, see here. In the NO, altars may be consecrated without these relics. So the question would be whether these altars are acceptable for use in the Traditional Mass or whether they would be required to be supplemented with a portable altar stone with relics. (Or the antimension of the East- a cloth with relics sewn into it)